The present invention relates generally to electronic musical instruments and, more particularly, to a method and apparatus for operating a keyboard electronic musical instrument, such as an electronic organ, for producing a novel vocal or chorale effect.
Vox or Vox Humana is the name of an effect produced by reed pipes of wind operated pipe organs wherein a human vowel-like sound is produced by a vibrating brass reed suitably coupled to an appropriate resonator. Many attempts have been made in the past to simulate or imitate this effect in a keyboard electronic musical instrument, e.g. an organ, it having been found that simple vowel-like sounds blend well with other organ voices and could be modulated using the same envelope signals normally produced by the instrument. The typical prior art approach to simulating such vowel-like sounds involves the use of multiple formant filters whose filtering characteristics are fixed to modify the harmonic content of an input tone signal in a particular manner. It is also known to simulate the Vox Humana effect by applying a harmonically rich tone signal waveform to a sharp cutoff low-pass filter, the cutoff frequency of the filter being automatically adjustable from a lower frequency when no keys of the organ are played to a higher frequency when a key is played. In yet another prior art electronic organ, the Vox Humana effect is simulated by applying a repeating series of pulse groups separated from each other and each containing a short sequence of two or more pulses to a sharp cutoff low-pass filter.
The Vox Humana effect or vowel-like sounds produced by prior art electronic organs tend to loose their distinctiveness and become monotonous after a time since the identically voiced steady-state tones produced by the fixed formant filters are rapidly interpolated by the ear even though they may be changed in pitch. It is, therefore, a basic object of the present invention to provide an electronic musical instrument capable of generating vowel-like sounds reminicent of the Vox Humana effect, which vowel-like sounds are produced in a manner extremely pleasing to the ear and are closely imitative of a vocal or chorale performance. It is a related object of the invention to provide an electronic keyboard musical instrument capable of generating vowel-like sounds when change in a dynamic manner according to a predetermined sequence.